The present invention relates to an automatic speed control device, particularly relates to an automatic speed control device suitable for controlling a running speed of a vehicle at a constant speed. The present invention further relates to an automatic speed control device suitable for performing a constant speed control of a prime mover in which load fluctuation is large, for example, a stationary emergency generator.
An automatic speed control device, particularly an automatic speed control device for controlling a speed of an automobile or vehicle at a constant speed has variously been considered. FIG. 1 is a diagrammatic view showing one embodiment of a conventional automatic speed control device for a vehicle. Reference numeral 1 is a speed signal detector for generating a pulse signals of a frequency proportional to a speed, reference numeral 2 is a speed signal generator for converting a pulse signal received from the speed signal detector 1 into a DC voltage signal proportional to a speed by an F-V converter (frequency-voltage converter), reference numeral 3 is a pulse generator for generating pulse signals of a certain or constant frequency, reference numeral 4 is a gate circuit for passing through the pulse signals to a counter 6 by turning on a set switch 5, and reference numeral 7 is a D-A converter for converting a counted result at the counter 6 into an analog voltage. During the turning on of the set switch 5, an output of the D-A converter 7 is compared with an output of the speed signal generator 2 in a comparator 8, and when both the outputs coincide with each other, the gate circuit 4 is closed by an output of the comparator 8 so as to stop the counting of pulse signals and the speed at the time is stored in the counter 6. Thereafter, a difference between the voltage output proportional to the actual speed from the speed signal generator 2 and the output generated by converting the set speed stored in the counter 6 into an analog voltage signal is compared and amplified in an error amplifier 9, further amplified by a power amplifier 10, and then supplied to a current-vacuum converter 11, so that the magnitude of a current from the amplifier 10 is converted into the magnitude of a vacuum from a vacuum source 12. The output of the current-vacuum converter 11 is supplied to a throttle valve 15 of a gasoline engine through a vacuum force converter 13 and a link mechanism 14, so that the valve 15 is opened or closed by the link 14 thereby to control the speed of a vehicle constant.
The above conventional device, however, has the following drawbacks.
(1) Since an F-V converter is used for the speed signal generator 2 and a D-A converter is used for converting a counted result of the counter 6, the speed measuring accuracy of a vehicle is insufficient.
(2) Because of a response characteristics of the F-V converter, when a speed is particularly slow and the frequency of a speed pulse is low, it is impossible to obtain a quick and precise response.